One of the devices intended to protect a vehicle's occupant against injury is an instrumentality such as an inflatable airbag which is housed in the passenger compartment forwardly of the occupant in a deflated condition. When such vehicle is subjected to acceleration of the kind accompanying a crash, the airbag inflates to provide a protective cushion for the occupant.
When a moving vehicle becomes involved in a crash of such severity as to slow or stop the vehicle suddenly, an occupant of the vehicle will continue forward movement at the rate the vehicle was traveling at the time of the crash until such time as the occupant collides with some portion of the structure of the passenger compartment. If serious injury to the occupant is to be prevented, the airbag must be fully inflated before a collision occurs between the occupant and some structure of the passenger compartment. It has been found that, typically, the driver of a vehicle can move forwardly only about five inches from the driving position before a restraining device such as an airbag is needed to prevent injury. All airbag inflating mechanisms require a period of time, such as thirty milliseconds, to effect inflation of the airbag once a signal initiating inflation has been received from a sensor. Under these circumstances, an airbag protective device, to be effective, must become inflated thirty milliseconds before the occupant has moved five inches from the position he occupied at the time of the crash.
It has been determined that a passenger protective device is needed when the movement of the vehicle occupant relative to the vehicle is at a velocity of about twelve miles per hour. Thus, an ideal sensor is one which will determine that the occupant will collide with some portion of the vehicle's interior at a speed of twelve miles per hour or greater, and thirty milliseconds before the occupant has moved forwardly five inches.
Since an airbag protective device is one which is designed to prevent forcible collision between a vehicle occupant and some portion of the passenger compartment, it is important that the sensor which initiates operation of the airbag inflation mechanism be one which is responsive to the acceleration of the passenger compartment, rather than some other part of the vehicle. Not all crashes, however, result in acceleration of the vehicle's passenger compartment to a degree necessary to require operation of the airbag. For example, if a front fender or bumper of a vehicle should collide with a pole or the like which breaks upon or shortly after impact, a sensor mounted on the front fender or bumper could experience a velocity change of twelve miles per hour, or greater, before the pole breaks, whereas the passenger compartment of the vehicle might experience a negligible velocity change. Under these circumstances, inflation of the airbag would not be required and, if it were, it might even contribute to a subsequent accident. It is important, therefore, that the sensor or sensors with which a vehicle is equipped be located in such positions and be of such construction as to be predictive that the passenger compartment will undergo a velocity change necessitating passenger protection.
Frontal portions of the vehicle, i.e., bumpers, fenders, radiators, and the like, may undergo a substantial velocity change relative to the passenger compartment inasmuch as such frontal portions of a vehicle are capable of collapsing and absorbing energy. Thus, sensors mounted at frontal positions on a vehicle should have characteristics different from sensors mounted on its firewall, for example, in order to assure inflation of an airbag when required, but to avoid inflation of the airbag in those instances in which it is not necessary.
In those sensors which initiate inflation of an airbag via an electrical circuit which requires the closing for a finite period of time a normally open switch, care must be taken to ensure that the switch will remain closed, not only for the minimum time required to effect circuit completion, but also for a somewhat longer period of time so as to provide a factor of safety. In sensors which utilize a biasing mass to establish an acceleration threshold which must be exceeded before closing of the switch is possible, there is a likelihood that the biasing mass may move to a position in which the switch operating means is free to move toward switch closing position, but then rebound into the path of movement of the switch closing means and delay or prevent closing of the switch, or effect premature reopening of the switch. On the other hand, in some instances it may be desirable to delay closing of the switch for a predetermined period of time so as to avoid initiating operation of the airbag until sufficient time has lapsed to ensure that the circumstances necessitate airbag inflation.
Sensors constructed according to the invention are so designed as to enable them to be mounted in selected positions on a vehicle and sense velocity changes which require and do not require actuation of a passenger protective device.